THIS invention relates to a method and apparatus for testing installation quality in a grouted anchor system.
The invention is particularly, but not solely concerned, with so-called “metal tensioned systems”. The term “metal tensioned system” is used in this specification to refer to the class of products which includes prestressed ground or rock anchors and rock bolts, sometimes referred to as “roof bolts”, of both bar and strand type. Products of this type have been in widespread use for many years in civil engineering and mining applications, in the latter case typically for the purposes of roof support and consolidation.
A metal tensioned system has a tendon of bar or strand form which is anchored in a predrilled hole by a mechanical anchorage or grout and subsequently tensioned. The present invention is particularly concerned with grout-type systems making use of resin or cement grouts. In such systems, it will be understood that the performance of the metal tensioned system is largely dependent on the quality of the grout used to anchor the tendon and hence the bonding of the tendon to the material in which the hole is drilled. Poor performance may, for example, result from poor or non-uniform grout application, insufficient quantity of grout or poor mixing of the grout. Simple visual inspection of the protruding end of the metal tensioned system usually provides little or no true indication of the condition or quality of the installation and in particular the quality of the grout. To address this problem a number of non-destructive testing methods have been proposed to test the installation quality of the grouted system. Such methods have included acoustic testing, electrochemical testing and magnetic flux leakage testing. Of the known methods, the acoustic methods have proved to be the most promising.
One known device for implementing an acoustic technique is known as the Boltometer. In the use of this technique an ultrasonic pulse is applied to the protruding end of a metal tensioned system under test, and the magnitude of the echo signal reflected in that system is measured. From these measurements, a skilled person can theoretically obtain an indication of the quality of the installation.
The Boltometer technique does however have several serious shortcomings. One of these is the inability of the technique to analyse grout quality in strand or cable-type metal tensioned systems, or in non-linear metal tensioned systems such as “Shepherd's Crook”—type systems. Other shortcomings are the necessity to grind the protruding end of the tendon flat in order for testing to take place, the necessity to calibrate the apparatus for the particular tendon and rock type under consideration and the necessity for an acoustic coupling between the Boltometer transducer and the tendon. Given that the acoustic system is applied to the tendon in the vicinity of this coupling, the coupling must be appropriately resistant to the applied signal.
Another known, somewhat similar testing technique is that proposed by the National Cooperative Highway Research Project (NCHRP). In this technique, an impact is applied to, for instance, the protruding end of a rock or roof bolt under test and the reflected frequency spectrum for that bolt is monitored and used to provide an indication of the installation quality of the grout of that bolt.
This technique also has its shortcomings, notably a necessity to grind the end of the tendon flat to enable an accelerometer to be attached and the fact that the tendon diameter must in general be at least 25 mm. In mining applications, tendons of such large diameter are seldom used, so the NCHRP technique is of limited value in such applications. Also, there is a necessity for the accelerometer coupling to be impact resistant.
It is an object of the present invention to provide a method and apparatus which addresses the above-noted disadvantages of the known Boltometer and NCHRP techniques.